The hydrocracking process is used to upgrade heavy oil fractions or feedstocks, such as heavy atmospheric gas oil, atmospheric resid, and vacuum gas oil, obtained from crude oil to more valuable lower molecular weight or lower boiling products, such as diesel, kerosene and naphtha. The heavy oil fraction that is typically hydrocracked comprises hydrocarbon components boiling above 290° C. (550° F.) with at least 90 weight percent of the heavy oil fraction boiling above 380° C. (716° F.). The heavy oil fraction may also contain asphaltene and polycyclic aromatic (PCA) hydrocarbon components. A typical heavy feedstock has an initial boiling point above about 315° C. (600° F.) and a final boiling point below about 590° C. (1094° F.).
Hydrocracking is accomplished by contacting in a hydrocracking reaction vessel or zone the heavy feedstock with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield the upgraded products. The product upgrading is accomplished by cracking the larger hydrocarbon molecules of the heavy feedstock and adding hydrogen to the cracked molecules to yield lower molecular weight molecules.
The per-pass conversion across the hydrocracker reactor of the heavy feedstock depends on a variety of factors, including, for example, the composition of the heavy feedstock, the type of hydrocracking catalyst used, and the hydrocracker reactor conditions, including, reaction temperature, reaction pressure and reactor space velocity.
The hydrocracker reactor product is passed to a separation system that typically includes a fractionator or stripper that provides for separating the hydrocracker reactor product to yield at least one lower boiling conversion product and a fraction which comprises the portion of the heavy feedstock that is not converted to lower boiling products. The fraction of heavy feedstock that is not converted can include polycyclic aromatic (PCA) hydrocarbons and asphaltenes contained in the heavy feedstock and PCA hydrocarbons that are formed as side products during the hydrocracking of the heavy feedstock. The separated fraction of unconverted heavy feedstock may be returned as a heavy oil recycle feed to the hydrocracker reactor.
One problem that is sometimes encountered in the processing of certain types of heavy and aromatic hydrocracker feedstocks is that the higher severity hydrocracker reactor conditions needed to provide for a desired high conversion can result in formation of PCA hydrocarbon side products that accumulate in the heavy oil recycle stream of the process. Additionally, in order to achieve the desired conversion of certain heavy hydrocracker feedstocks, the rate of heavy oil recycle often needs to be higher than that typically required when processing other types of feedstock. The combination of the formation of PCA hydrocarbons and higher recycle rates can cause an undesirable buildup of PCA hydrocarbons in the heavy oil recycle stream. This buildup can cause numerous problems in the operation of a hydrocracking process, such as, for example, increasing the rate of catalyst deactivation, reducing conversion yields, and causing equipment fouling.
A number of methods have been proposed in the prior art to prevent the undesirable buildup of PCA hydrocarbons (also referred to as polynuclear aromatics or PNAs) in a heavy oil recycle stream. One such method involves taking a small bleed stream of a polynuclear aromatic compound-rich condensate of the reactor effluent and discarding it (U.S. Pat. No. 3,619,407). However, this approach will result in the loss of valuable lower boiling hydrocarbons, since the bleed stream containing unconverted hydrocarbons is discarded instead of being converted.
Another approach to solving the problem of PNA hydrocarbon build-up in the heavy oil recycle stream is disclosed in U.S. Pat. No. 4,447,315 which involves passing a heavy PNA-containing recycle stream produced by fractionating the liquid phase material recovered from the reactor effluent through a suitable bed of activated carbon or alumina. This method requires the disposal of PNA contaminated adsorbents which is relatively expensive and challenging from an environmental standpoint.
U.S. Pat. No. 4,698,146 discloses a hydrocracking process in which a large portion of the PNAs are recovered in the slack wax stream of a vacuum distillation unit which is said to make the bottoms stream from the vacuum distillation unit more suitable for upgrading in a solvent deasphalting unit. The low value PNA-containing slop wax stream is isolated from any subsequent introduction into the hydrocracking reaction zone, which results in the loss of some of the higher boiling hydrocarbons which are not converted.
Accordingly, there is a continuing need for an improved hydrocracking process that provides for the high conversion hydrocracking of heavy hydrocarbon feedstocks and the reduction of buildup of heavy poly-aromatic hydrocarbons in the heavy oil recycle stream of the hydrocracking process.